The present invention relates to control systems for DC electric powered vehicles and more particularly to a control system including a plug current limit circuit.
In DC electric powered vehicles, it is common practice to employ the drive motor to brake the vehicle by reversing the direction of torque of the motor. This type of braking is known as dynamic braking or plugging. Electric industrial vehicles are presently designed such that when running at full speed, a direction selector may be moved to select an opposite direction without releasing the accelerator. In such vehicles an electric control circuit overrides the accelerator demand and provides a substantially constant level of braking torque until the motor speed has dropped to near zero.
Prior art control systems regulated braking torque by maintaining a constant level of field flux or by maintaining a constant level of armature current. Both of these methods have known defects, a typical example of which is an effect called "cogging". Cogging is recognizable as one or more bumps or lurches of the vehicle which normally occur toward the end of the braking interval as a result of variations in braking torque. It has been found that cogging is much more pronounced when the vehicle is switched into a braking mode at a low rather than a high speed. It has also been found that cogging is more pronounced in those vehicles using electric motors with a high K.sub.v motor constant, which motors produce more counter EMF per ampere of field current. Prior art attempts to aleviate cogging by selection of motors having low K.sub.v motor constants have been inefficient and ineffectual. Other attempts to reduce cogging by providing controlled acceleration have proven impractical because excessive response times produce sluggish operation of the vehicle.
We have found that the prior art attempts to prevent cogging have been unsuccessful primarily because the prior art has failed to recognize the true cause of the problem. By experimentation and analysis, we have found that cogging is an instability or oscillation in the control circuit which is directly associated with the increased torque per ampere of field current as the motor speed decreases. For example, at a very high speed, very little field flux is required to produce the required level of armature current and therefore since the torque is proportional to the product of armature current and field flux the net torque is small. As the speed decreases, an increasing amount of field flux is required to maintain the desired armature current; therefore, the torque per ampere of armature current increases and intensifies the torque variations with decreasing speed.
The torque variations result from the control system being underdamped, so that switching into a current limit mode produces an initial overshoot oscillation and, at low speeds where the torque per ampere is high, these oscillators can be felt cogging. This oscillation or ringing when the armature current is suddenly stepped is greater and more pronounced in terms of motor torque at low speeds than it is in the higher speed regions. The overall performance of a control system with the characteristics which produce this overshoot is desirable in order to prevent sluggish vehicle performance. Consequently, we have found that the best solution is to reduce the effects of the overshoot rather than alter the control system constants.
It is an object of the present invention to provide a control system which overcomes the cogging effect.
It is a further object of the present invention to provide a control system which overcomes the cogging effect without sacrificing system response.